Prior art magnetic resonance imaging apparatus' generate a uniform magnetic field which is also referred to as the main field or the polarising field B0. The purpose of the magnetic field is that the individual magnetic moments of the spins in the tissue to be visualised attempt to align with this polarising field, but precess about it in random order at a characteristic Larmor frequency which is determined by the gyromagnetic constant of the spins and the polarising magnetic field B0.
However during operation of a magnetic resonance apparatus the magnetic field can vary over time due to various reasons. For example when the magnetic resonance apparatus is under heavy duty the passive iron shim pieces of the coils which generate the magnetic field heat up which leads to a fluctuation of the magnetic field.
U.S. Pat. No. 6,294,913b1 shows a method for compensation of variations in the polarising magnetic field during magnetic resonance imaging. Monitor signals are acquired in an interleaved manner during a scan with the MRI system. Frequency changes caused by variations in the polarising magnetic field B0 are measured using the monitor signals, and these measured frequency changes are employed to compensate image data acquired during the scan. This compensation is achieved by changing the frequency of the RF transmitter and receiver to offset the effects of changes in B0. A disadvantage of this compensation method is that special monitor signals are required for the compensation.
It is therefore an object of the present invention to provide for an improved method of monitoring and compensating a magnetic field drift.